KVM: arm64: Correctly plumb ID_AA64PFR2_EL1 into pkvm idreg handling

KVM: arm64: Correctly plumb ID_AA64PFR2_EL1 into pkvm idreg handling

While we now compute ID_AA64PFR2_EL1 to a glorious 0, we never use
that data and instead return the 0 that corresponds to an allocated
idreg. Not a big deal, but we might as well be consistent.

Reviewed-by: Sascha Bischoff <sascha.bischoff@arm.com>
Fixes: 5aefaf11f9af5 ("KVM: arm64: gic: Hide GICv5 for protected guests")
Link: https://sashiko.dev/#/patchset/20260319154937.3619520-1-sascha.bischoff%40arm.com
Link: https://patch.msgid.link/20260401103611.357092-15-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>